A cyclic variation in yields of infectious virus was observed when standard and SSPE strains of measles virus were serially passaged undiluted. Cyclic infectivity was observed even though virus was plaque purified before serial passage. This cyclic activity is like caused by defective interfering measles virions. Direct proof of this hypothesis, however, has not been obtained although a variety of ultracentrifugation techniques and polyacrylamide gel electrophoresis have been tried.